Position-coding patterns are known which have the property that the absolute position in the position-coding pattern for a partial area of the position-coding pattern of a smallest predetermined size can be calculated by means of the information in the partial area and a predetermined algorithm. Examples of such position-coding patterns are disclosed in U.S. Pat. No. 5,477,012 (O. Sekendur), where each position is coded by means of a symbol, and WO 92/17859 (Hewlett Packard Co.), U.S. Pat. No. 6,570,104 (Anoto AB), U.S. Pat. No. 6,674,427 (Anoto AB), U.S. Pat. No. 6,667,695 (Anoto AB), US 2003/0012455 (Anoto AB) and U.S. Pat. No. 5,973,110 (Xerox Corp), where each position is coded by means of a plurality of symbols.
Position-coding patterns of this type can be used, for instance, for digital recording of information which is written and/or drawn by hand by means of a user unit on a base provided with the position-coding pattern. While moving the user unit, images of the position-coding pattern are continuously recorded locally at the tip of the user unit. One or more symbols in each of the images are decoded to a position. The decoded positions together constitute a digital description of the user unit's movement over the base.
The position-coding pattern in the recorded images can be decoded in different ways. For instance, aforesaid US 2003/0012455 discloses a decoding method which involves calculating probabilities that symbols which are identified in a recorded area of the position-coding pattern assume any of a plurality of different possible decoding values. These probabilities are stored as real (floating precision) numbers in probability matrices and are used together with the different possible decoding values for decoding the identified symbols into coordinates for positions in the position-coding pattern.
Aforesaid U.S. Pat. No. 6,674,427 and U.S. Pat. No. 6,667,695 disclose other ways of decoding or determining a position from a recorded image of a position-coding pattern.
The known position decoding methods may be regarded to have the common features that symbol data is extracted from an isolated image in a sequence of images to be decoded; that position data is derived by effecting one or more lookup operations, using the symbol data, in one or more data structures that contain fundamental coding data of the position-coding pattern; and/or that the symbol or position data is used as input data to a mathematical formula for explicit calculation of an absolute position.
Applicant's pending U.S. application Ser. No. 10/282,001, which was not yet publicly available at the date of filing of the present application, but later published as US 2003/0128194, discloses a method for position decoding, where a position for a recorded area of the position-coding pattern is calculated in a known manner, for instance in accordance with aforesaid US 2003/0012455. However, in case a decoding error is detected for said position, either for the position as a whole or for one of its coordinates, an additional operation is performed as “a last resort”; information determined from the recorded area is compared or matched with information about how the position-coding pattern is composed in a matching area around a successfully decoded nearby position. The best matching portion of the known matching area is selected, and a corresponding position is determined as the decoded position. Thus, in summary, US 2003/0128194 discloses mandatory use of decoding by calculation of position from recorded area, followed, in case of a decoding failure, by an additional matching operation.